This invention relates to a filtering device for a molten metal such as molten aluminum, and more particularly to a molten metal filtering device so constructed that the filter medium of porous ceramic material in the shape of a flat plate can be changed without difficulty and it can be prevented from being clogged either owing to the incoming of the oxide film on the surface of the molten metal or owing to the solidification of the molten metal due to the drop of temperature of the filter medium itself.
In recent years, there have been proposed molten metal filter media which are made of porous ceramic material and formed in the shape of flat plates. The filter media of this kind have large capacities for filtration and are able to provide effective treatment of successive lots of molten metal produced in the smelting furnace. However, considerable problems have occurred in the conventional molten metal filtering devices fitted with the filter medium of porous ceramic material in the shape of a flat plate.
In the conventional filtering devices, molten metal flows downwardly to the filter medium. When the inflow of the molten metal from smelting furnaces to the filtering device is stopped during switching from one smelting furnace to another or after the end of the filtering operation, the molten metal left in the filtering device flows away from the device through the filter medium. For this, the filter medium is exposed in the ambient air. In this time, the filter medium may become clogged because the oxide film, such as aluminum oxide film, which is formed on the surface of the molten metal contacted with the ambient air in the filtering device, flows into the filter medium with the remaining molten metal and sticks on the medium. The filter medium may also be clogged because the molten metal which sticks on the medium or is left in the medium is solidified due to the drop of temperature of the filter medium exposed in the air.
The conventional molten metal filtering devices, especially those used for filtering molten metal produced by a plurality of successively operated smelting furnaces, operated do not give sufficient consideration to the problem of solidification of molten metal in the filter media which occurs while the devices are maintained during switching from one smelting furnace to another, as well as to the problem of damage done to the filter media due to such solidification.
Moreover, in the conventional filtering device, the used filter medium must be changed to a new one when it is exposed in the air and cooled to a lower temperature. When the conventional filtering devices are actually put to use, the filter media incorporated therein cannot be easily changed at the elevated temperatures at which the devices are operated.
The batchwise operation which comprises the steps of fusing a metal, especially aluminum in the form of ingots, in a smelting furnace, filtering the produced molten metal with a filter medium, and casing the filtered molten metal and producing slabs or billets has found popular acceptance. Recently, with a view to enhancing the productivity of this operation and ensuring the economization of energy, efforts are being made to shorten the intervals between the successive batchwise operations of the smelting furnace. When the intervals between the batchwise operations of the smelting furnace are to be shortened, no time can be spared for allowing the temperature of the filter medium and that of the filtering device using the filter medium to fall sufficiently. Consequently, the work for changing the filter medium must be carried out while the filter medium is kept at its elevated temperature. This work, therefore, proves to be dangerous. Furthermore, in the conventional filtering device in which a filter medium made of porous ceramic material and not readily wetted with the molten metal is arranged, occurs a lag occurs between the time the molten metal reaches the upper side of the filter medium exposed in the air and the time the molten metal begins to pass the filter medium. Therefore a head greater than the head of the molten metal during the filtration is required.
Thus, the development of a device which permits easy and safe changing of the filter medium has long been in demand. Moreover, in order to fully utilize the advantageous properties of the filter medium made of porous ceramic material, i.e. large capacity for filtration and effective treatment of successive lots of molten metal, there is a need for the development of a device capable of lessening the frequency of changing such filter media.